Manufacture of diborane by electrolysis of metal borohydrides



March 3, 1959 R. K. BiRDWHlSTELL EI'AL 2,

MANUFACTURE OF DIBORANE BY-ELECTROLYSIS OF METAL BOROHYDRIDES Fil ed Aug; 21, 1956 PLATINUM ELECTRODES v I I (/ll:

I I I '1' 1 STOPCOCK STOPCOCK L PLATINUM "ELECTRODE PLATINUM 1 ELECTRON-2" I i WELLV DOUGHNUT RA? K. BIRDWHISTELL E UILL. m 4? 2 m? R erronugvs United States Patent MANUFACTURE OF DIBORANE BY ELECTROL- YSIS 0F METAL BOROHYDRIDES Ralph K. Birdwhistell, East Lansing, Mich, Harry E. Ulmer, Lynn, Mass and Laurence L. Quill, East Lansing, Mich., assiguol's, by mesue assignments, to Olin Mathieson Chemical Corporation, a corporation of Virginia Application August 21, 1956, Serial No. 605,327

SCIaims. (Cl.204-59) This invention relates to a method for the manufacture of diborane by the electrolysis of metal borohydrides in dialkylformamides.

Diborane is diflicult and expensive to produce in quantity because of its instability, extreme reactivity and rapid hydrolysis. One of the best known processes is the reaction of lithium hydride with an etherate of boron trifluoride. Many diflicult engineering operations are encountered, however, in attempting to operate this process on a commercial scale. Another proposed process is based on thereaction of a metal borohydride in an anhydrous ether with ferric chloride. A disadvantage of this process is that extremely large quantities of solvent are required to obtain an economically feasible rate of reaction. The are process produces diborane by the reaction of hydrogen and boron trihalide in an arc discharge. This process has the disadvantages that (1) the halodiboranes are produced simultaneously with the desired diborane and they are impossible to separate by conventional engineering methods; (2) the process is inconvenient to operate, and (3) expensive liquid nitrogen is required in the separation of the diborane. The above methods utilize costly raw materials and in many cases are not practical for the production of commercial quantities of diborane.

The present invention provides a process in which diborane is manufactured electrolytically. The process comprises passing a direct electric current through a solution of a metal borohydride in a dialkylformamide. The electrodes can be of any suitable material. For example, both can be of platinum or the process can be carried out using a mercury cathode. When an alkali metal borohydride is electrolyzed with a mercury cathode, diborane and hydrogen are released at the anode. The alkali'metal discharged at the cathode combines with the mercury to form an amalgam which is removed from the cell as a valuable product. The alkali metal can be recovered therefrom in known ways, for example, as the caustic alkali or as an alcoholate and the denuded amalgam can be recycled to the cell.

The present invention enables one to produce diborane in pure form with high yields and in quantities limited 2 terial, other alkali metal borohydrides including lithium and potassium borohydrides can be used. In addition, other metal borohydrides sufficiently soluble in the dialkylformamides, for example, calcium borohydride, magnesium borohydride and aluminum borohydride can be used.

N,N-dimethylformamide is readily available commercially and is preferred since it is the best solvent for the metal borohydrides. However, other N,Ndialkylformamides can be used including N,N-diethylformamide, N,N- dibutylformamide and other N,N-dialkylformamides, preferably containing not more than about 5 carbon atoms in each alkyl group. If desired, mixed alkyl groups may be used, for example, N-methyl-N-ethylformamide.

When sodium borohydride is electrolyzed in N,N-dimethylformamide using platinum electrodes, BH; ions are discharged at the anode and the radicals decompose into V2 H, and A B H At the cathode Na+ ions are discharged, and react with sodium borohydride or its components to liberate /2 H The mixed anode and cathode gases thus comprise 2 volumes of hydrogen and one volume of diborane.

In carrying out the invention in one of its various forms a saturated or nearly saturated solution of sodium borohydride or other metal borohydride in dialkylformamide is maintained in the cell. Provision can be made for continuous introduction of solid borohydride to a portion of the cell or the electrolyte can be circulated to a saturator and returned to the cell. During electrolysis diborane and hydrogen are produced at the anode and pass out of the cell into low temperature condensers where the diborane is condensed and collected. Partially spent electrolyte to which additional metal borohydride has been added can be recycled to the cell after being filtered. Sodium or other amalgam which forms at the cathode is removed from the cell and can be converted to the hydroxide or various other valuable compounds, or the sodium or other metal can be extracted from the amalgam, converted into the borohydride, and recycled to the operation. To maintain a high cell efliciency any water present in the mercury recovered from the amalgam should be removed before the mercury is recycled to the cell; no chemical treatment is necessary to accomplish this since the water may be removed by a simple mechanical separation.

The concentration of the metal borohydride in the electrolyte can vary from a slurry of borohydride in its saturated solution to any lower concentration which is consistent with economical operation; the preferred cononly by the raw materials and equipment available. This process avoids the diflicult refining steps usually required to produce maximum purity diborane by other processes. The anodic product is a simple mixture of hydrogen and diborane from which the latter can be readily separated in a high degree of purity. Another important advantage is that only one chemical substance, the metal borohydride, enters into the electrochemical reaction to produce diborane in a process that is convenient to operate and which presents no unusual engineering problems when operated on a large scale.

The alkali metal borohydrides referred to are stable commercial products which are conveniently handled. Both the solvent and the alkali metal borohydride introduced into the cell should be anhydrous since any water present in the electrolyte will result in a decreased yield.

While sodium borohydride is the preferred starting macentration of borohydride is from 5 percent by weight of borohydride to approximate saturation in the electrolyte. For most efiicient operation, the electrolyte should be saturated with diborane.

The cell operates safely over a wide range of amalgam composition. metal that can be tolerated in the mercury is reached with the viscosity of the amalgam such that it will not flow by gravity. With the preferred method of operation sodium in the amalgam is permitted to vary from 0.01 to 0.50 percent by weight.

A voltage range of approximately 5 to 10 volts is preferred in the operation of this cell. In the presence of impurities higher voltages are observed during which time less than the optimum quantities of diborane are produced. The typical operation in the voltage range suggested results in current densities of about one or two amperes/sq. inch.

As those skilled in the art will readily understand, the temperature at which the process is carried out can be varied widely, temperatures within the range from about 15 to C., for example, being suitable. For instance, excellent results can be obtained when operat- The maximum value of sodium or other ing at 60' to 70' C. and with voltages from to 7 volts. External heating or cooling may be resorted to as desired.

Example I The electrolysis cell of the accompanying drawing was formed from a glass U-tube with sidearms carrying stopcocks. Platinum electrode leads were introduced through ground glass joints capping the main arms of the U-tube. The electrodes were separated at the bottom of the U- tube by a vertically arranged glass doughnut which divided the cell into two compartments and effectively separated the anode and cathode gases. A small well arranged as an appendix in one branch of the U-tube just above the bottom contained the sodium borohydride. The solid was thus in contact with electrolyte and maintained it saturated with solute.

Approximately 3 grams of sodium borohydride (recrystallized from water) was introduced into the well, the apparatus was evacuated and about 21 grams of N,N- dimethylformamide was distilled into the cell. Electrolysis was conducted by passing a current of 0.6 amperes at 210 volts. When the solid sodium borohydride. was consumed, the flow of current ceased. At this time the temperature of the electrolyte had risen from room temperature to about 55 C. Approximately two volumes of anode gas and one volume of cathode gas were combined and passed through cold traps at 78 and -l96 C. The trap at -78 C. removed dimethylformamide carried over by the gases. The residual gas was separated into one volume of diborane, liquefied in the -l96' C. trap and two volumes of hydrogen, uncondensed in the -l96 C. trap. The diborane reduced permanganate solution and gave a positive reaction with the silver nitrate-amylamine reagent.

Example 11 Using a mercury cell, grams of sodium borohydride dissolved in about 150 grams of dimethyltormamide is electrolyzed at C. using a current density of about two amperes per square inch at 10 volts. The anode gas is collected and separated by fractional condensation to obtain diborane and hydrogen. Sodium dissolves in the mercury and is removed as amalgam. The latter is denuded by reaction with water in contact with quarter inch graphite granules to form aqueous caustic and hydrogen. The sodium-depleted amalgam is dried and returned to the cell.

It is claimed:

1. A method for the production of diborane which comprises passing a direct electric current through a substantially saturated solution of a metal borohydride in a lower N,N-dialkylformamide and recovering diborane as an anode reaction product.

2. The method of claim 1 wherein said metal borohydride is an alkali metal borohydride.

3. The method of claim 1 wherein said metal borohydride is sodium borohydride.

4. The method of claim 1 wherein said lower N,N- dialkylformamide is N,N-dimethylformamide.

5. A method for the production of diborane which comprises passing a direct electric current through a sub-- stantially saturated solution of sodium borohydride in N,N-dimethylformamide while the solution is maintained at a temperature within the range from 15 C. to C. and recovering diborane as an anode reaction product.

OTHER REFERENCES Transactions of the Electrochemical Society, vol. 63 (1933) pages 231 through 238, article by Blue et al. 

1. A METHOD FOR THE PRODUCTION OF DIBORANE WHICH COMPRISES PASSING A DIRECT ELECTRIC CURREENT THROUGH A SUBSTANTIALLY SATURATED SOULATION OF A METAL BOROHYDRIDE IN A LOWER N,N-DIALKYLFORMATION AND RECOVERING DIBORANE AS AN ANODE REACTION PRODUCT. 